The invention relates to the electrodeposition of zinc, and particularly to a plating bath for plating bright level zinc deposits from aqueous acid plating baths. More particularly, the invention relates to the incorporation in the acid zinc baths of at least one bath-soluble aromatic sulfonic acid or salt wherein the aromatic group preferably contains one or more lower alkyl group. The invention also relates to methods for electrodeposition of level and bright zinc deposits from such baths.
Considerable attention has been directed over the years to the development of zinc electroplating baths which will produce bright and level deposits of improved quality. Much research has been devoted to improving the overall brightness, the range of allowable current densities, and ductility of the zinc deposit. Until recently, most of the successful zinc plating baths were aqueous alkaline zinc plating baths containing substantial quantities of cyanide which has caused concern regarding toxicity and waste disposal problems.
Accordingly, activity in the plating area has been devoted either to the development of cyanide-free alkaline plating baths or improvements in acid plating baths. This invention relates to acid plating baths.
Typically, acid plating baths have been based on a suitable inorganic zinc salt such as zinc sulfate, and the baths usually include buffers such as the corresponding ammonium salt and other additives to promote and improve ductility, brightness, throwing power and covering power. Surface active agents may be included to improve crystal structure, reduce pitting, and increase the solubility of the other additives.
U.S. Pat. No. 4,076,600 is an example of a patent relating to acid zinc plating baths and the invention therein relates to the use of compositions containing phosphorus cations as leveling agents. The baths also may contain small amounts of nitrogen-containing compounds obtained by reacting (a) ammonia, an aliphatic amine containing at least one primary amine group, or mixtures of two or more of any of these with (b) one or more epihalohydrins, glycerol halohydrins or mixtures thereof. The use of bath-soluble reaction products obtained by the reaction of a nitrogen-containing heterocyclic compound with an acyclic amine having at least two functional groups separated by at least one different group, formaldehyde and an epihalohydrin of glycerol halohydrin in alkaline zinc electroplating baths is described in U.S. Pat. Nos. 3,655,534 and 3,849,325.
Aromatic carbonyl-containing compounds generally are incorporated into acid zinc baths as a supplemental brightener additive and for improving the fineness of the grain of the zinc deposit. Wetting agents or surfactants have been added to these baths to solubilize or improve the solubility of the carbonyl-containing compounds in the bath, but such wetting agents and surfactants generally result in a bath exhibiting a tendency to foam excessively, particularly on agitation and at the higher current densities often used in zinc plating.
U.S. Pat. No. 4,075,066 describes ammonia free zinc plating baths containing polyoxyalkylated naphthols, aromatic carboxylic acids or salts and at least one aromatic sulfonic acid or salt. A variety of aromatic sulfonic acids are suggested and one group of these are the water soluble salts of tetrahydronaphthalene sulfonic acid.